*4.5. Optical Characteristics*

Colour measurement and opacity estimates were used to conduct an optical investigation of the Ch-EPPE films. Using a Miniscan EZ colorimeter (HunterLab), the values of the Hunter colour (L\*, a\*, and b\*) were calculated. Equation (2) was used to calculate the film's whiteness index using these values [57]. Six readings were taken at various locations on each film. Five film samples were utilized in each replication, which involved five replications for each treatment.

$$\text{Whiteness index } = 100 - \sqrt{\mathbf{a}^2 + (100 - \mathbf{L})\mathbf{2} + \mathbf{b}^2} \tag{4}$$

Utilizing the technique developed by Rambabu et al. [37], the opacity of the film was determined by measuring the absorbance of a rectangular film sample (1 × 4 cm in size) using a UV-Vis spectrophotometer (UV-3600, Shimadzu, Baltimore, MD, USA) with an absorbed wave length of 600 nm. The film's opacity was calculated using Equation (5).

$$\text{Operity} = \frac{\text{Absorbance at 600 nm}}{\text{Fillm thickness (mm)}} \tag{5}$$

High transparency values, based on this equation, indicate poorer transparency and a greater level of opacity.
